Good afternoon, everybody. Earlier today, I was briefed by my homeland security team on the events in Boston. We're continuing to monitor and respond to the situation as it unfolds. And I've directedMore >>

President Barack Obama's statement on the explosions at the finish line at the Boston Marathon.More >>

BOSTON (RNN) - While people fight for their lives at local hospitals, the search is on for possible motives and suspects in Monday's deadly bombing at the Boston Marathon.

At least three people are dead and more than 140 are injured after back-to-back explosions near the finish line of the race held annually on Patriots Day.

Of those injured, at least 17 were critical, according to CNN. Some victims arrived at hospitals without limbs. Others required amputations and needed several surgeries in the coming days. Doctors pulled ball bearings out of wounds.

An 8-year-old boy, Martin Richard, died in the blast, and his mother and sister are seriously wounded, according to the Boston Globe.

Officers provided no further details about the fatalities.

No group has claimed responsibility for the attack.

Authorities searched an apartment building in the Boston suburb of Revere, CNN reported, but it has not been confirmed whether the search has any connection to the bombings.

Michael McCaul, chairman of the Homeland Security Committee, told CNN authorities are questioning a person of interest but no suspect has been named.

The FBI has taken over the investigation through the Boston joint terrorism task force, according to FBI Special Agent in Charge Rick DesLauriers.

Police searched for a Penske rental truck, the driver of which tried to gain access to the blocked-off area earlier in the day.

Boston Police Commissioner Ed Davis would not confirm a person of interest, only saying "there are people we are talking to."

Davis said the explosions happened 50 to 100 yards apart at about 2:50 p.m. ET, nearly two hours after the first of the event's winners crossed.

Weary runners who had already gone 26 miles had to run away from explosions as police officers urged people to flee the area.

"This cowardly act will not be taken in stride," Davis said. "We will turn every rock over to find the people responsible for this."

Dr. Peter Fagenholz, a doctor at Massachusetts General who performed six surgeries Monday, said the majority of injuries are "combined, complex lower extremity" injuries.

"I've never seen this volume come in this quickly," he said.

Fagenholz credited his staff for rising to the occasion.

"We've had as much or more manpower, people power, than we can use," he said.

Brigham and Women's Hospital saw similar injuries in 31 patients they treated.

"I don't know if you can ever prepare for something like this, but if you can, we were," Dr. Ron Walls told CNN. "Every day, we take care of patients with bad trauma, and we have drilled for things like this."

Speaking from the White House, President Barack Obama promised those responsible "will feel the full weight of justice," but stopped short of calling it a terror attack.

"We still do not know who did this or why, and people shouldn't jump to conclusions before we have all the facts," he said. "We will find out who did this and why they did this."

Reuters said two high-level U.S. law enforcement sources told them one or more bombs were responsible for the detonations. Intelligence officials told the Associated Press at least two more devices had been discovered by police and were being dismantled.

"There are a number of parcels that have been dropped by people along the race route," Davis said at a 5 p.m. Monday news conference. "Anything that's out there right now is being treated as a suspicious device ... they may be blowing things up over the next few hours but right now, we have not found another suspicious device."

The area has been closed and secured as a crime scene by the National Guard.

A fire was also reported at the JFK Presidential Library and Museum at roughly 4:20 p.m. Police say the incident "appears to be fire-related," according to a tweet from Cheryl Fiandaca, public information officer for Boston PD. No injuries were reported.

A controlled explosion of another possible bomb had been done by police near Boylston Street, where the first two explosions took place, Davis added.

Bob O'Donnell, a firefighter from Stoughton, 20 miles south of Boston, was watching his son race and saw many of the injured.

"It's an awful thing, right at the finish line when the average Joes were just coming in," he told Boston affiliate WCVB. "I'm concerned because there were a lot of mothers, fathers and whole families involved there, they're all standing together.

"People tried to get them away because they weren't seriously injured, but they wouldn't leave their loved ones, and I don't blame them."

Mary Lou Tooley from Wilmington, NC, was a block away in the family meeting area for the marathon when the explosion happened.

"The first explosion went off and the entire area went silent," Tooley said. "Then the second explosion went off, I'd say maybe two minutes after that and everything went silent again. That's when the ambulance and the police cars came you know went to the scene, clearing the scene."

Cell phone service in the area was shut down to prevent any possible devices from going off, according to Boston news affiliate WBZ.

The FAA has restricted airplanes from flying over the site until further notice. They also issued a ground stop for planes at Boston's Logan Airport. It has since been lifted.

The Lenox Hotel, located a block from the site, also was evacuated.

Boston is on a heightened state of alert, and people in the area have been informed by Boston PD that they should expect a beefed-up police presence.

As a precaution, both Washington DC and New York City went on a heightened state of alert.

"Shortly after being notified of the incident around 3 p.m. EDT, the president received a briefing from Homeland Security adviser Lisa Monaco and other members of his senior White House staff in the Oval Office," the White House said in a statement.

"The president called Boston Mayor Tom Menino and Massachusetts Gov. Deval Patrick to express his concern for those who were injured and to make clear that his administration is ready to provide needed support as they respond to the incident."

The Boston Marathon released a statement extending "deepest sympathies to all those who were affected."

"What was intended to be a day of joy and celebration quickly became a day in which running a marathon was of little importance," the statement read.

The NHL postponed the Boston Bruins' Monday hockey game against the Ottawa Senators. It will be rescheduled at a later date.

The Boston Celtics canceled Tuesday's basketball game against the Indiana Pacers. It won't be rescheduled.